The present disclosure relates to a thermal transfer sheet, particularly, to a component forming a heat resistant smooth layer provided in the thermal transfer sheet.
A thermal transfer method using a sublimation dye is employed for reproducing a full-color image by color dots of a multiple of colors by transferring the multiple of color dots onto a material to which the color dots are to be transferred by heating for a remarkably short time. In the thermal transfer method, a so-called sublimation type thermal transfer sheet in which a dye layer including the sublimation dye and a binder is provided on one surface of a base sheet such as a polyester film is used as a thermal transfer sheet.
Further, in the thermal transfer method, the thermal transfer sheet is heated from a back thereof by a heating unit such as a thermal head according to image information, and an image is formed by transferring a dye contained in the dye layer onto a material (printing paper) to which the dye is to be transferred. It is necessary that a friction between a surface of the thermal transfer sheet contacting the thermal head and the thermal head is stably low from a low density printing to a high density printing. Therefore, in the thermal transfer sheet in general, a heat resistant smooth layer is provided on a surface which is the other side of the surface on which the dye layer is formed for the purposes of preventing fusion with the thermal head and imparting good running smoothness (smoothing property).
For a heat resistant smooth layer, a method of forming a layer formed of a thermally crosslinkable resin is often employed for the purpose of imparting heat resistance to a base sheet, and it is possible to impart abrasion resistance and heat resistance to a thermal transfer sheet by the method. Though it is possible to obtain the heat resistant smooth layer having good heat resistance by employing the method, it is difficult to impart smoothness only by the thermally crosslinkable resin binder. Further, since the process is complicated due to the necessity of performing an aging treatment (heat curing) at 50° C. for about one week for the thermal crosslinking, a considerably long production time is incurred in the end.
As a method for solving the above-described issue, a method of using a mixture of polyamideimide and polyamideimide silicone as a binder has been proposed (see Japanese Patent Application Laid-Open No. 2001-334760, for example). In the case where the above-described binder is used, since the polyamideimide and polyamideimide silicone have Tg of 200° C. or more, it is possible to impart heat resistance to the thermal transfer sheet. Further, in the case where the binder is used, since it is possible to impart smoothness by an action of a silicone unit of the polyamideimide silicone, it is possible to obtain a good heat resistant smooth layer without heat curing.
Further, as another method for solving the issue, a method of using a cellulose acetate butyrate resin as the binder for the heat resistant smooth layer has been proposed (see Japanese Patent Application Laid-Open No. 2008-105373, for example). In Japanese Patent Application Laid-Open No. 2008-105373, a method of adding an acryl silicone resin is exemplified in Examples.